Semiconductor nanocrystals are photostable fluorophores with narrow emission spectra tunable through visible and near-infrared wavelengths, large molar extinction coefficients, and high quantum yields. These properties make semiconductor nanocrystals powerful tools for labeling and optical sensing in biological, biomedical, and environmental contexts. The exceptional brightness and photostability of semiconductor nanocrystals give them great potential for analyzing biological events at the single molecule level. However, current nanocrystals designed for cellular labeling applications suffer a tradeoff between size, non-specific binding, and derivatizability.
Nanocrystals generally include an inorganic nanoparticle that is surrounded by a layer of organic ligands. This organic ligand shell is critical to the nanocrystals for processing, binding to specific other moieties, and incorporation into various substrates. Nanocrystals can be stored in their growth solution, which contains a large excess of ligands such as alkyl phosphines and alkyl phosphine oxides, for long periods without noticeable degradation. For most applications, particularly aqueous applications, nanocrystals must be processed outside of their growth solution and transferred into various chemical environments. However, nanocrystals often lose their high fluorescence or become irreversibly aggregated when removed from their growth solution.